This invention relates to active force transducers, and more particularly to a beam style active force transducer, and its control and measurement electronics as for high speed high accuracy weighing of material in a production operation, e.g., of filling powders and liquids into containers, or determining the mass weight of an object.
This invention arose from a recognition that modern high speed manufacturing operations, as for assembling small fill containers, would greatly benefit by the capability of rapidly measuring, in a small fraction of a second, the weight of an individual substance or material to be included or incorporated into the assembled product, and to do so with highly accurate results. Such capability would include that of repeatedly measuring the tiny quantities on a production basis.
In order to increase efficiency and the quality of products manufactured on assembly lines, manufacturers are searching for automated methods of monitoring the manufacturing operations as well as the products themselves. As technology advances, assembly line production rates are increasing, requiring more advanced and faster transducers. Commercially available units have a range limit of about 600 grams, plus or minus 50 milligrams, for weight checking.
The accuracy, response time and range are the main characteristics that determine the suitability of a transducer for a particular application. Accuracy and response time are typically functions of the range. In addition, since the potential exists for mechanical vibrations to significantly reduce the achievable accuracy of transducers in manufacturing environments, it is important to design transducers with vibration disturbance rejection.
Thus, a force transducer system with a fast response time, a high level of accuracy, and vibration disturbance rejection characteristics is desirable. The target parameters for such a force transducer are accuracy up to 0.04% of full scale 0.3N, accurate measurement in 0.05 seconds, and accuracy minimally affected by vibrations.
There are many different methods and devices used for measuring force, of which the following are representative of the basic underlying principles for most force transducers. Almost all sensing elements designed to measure force utilize one of two basic principles relating to "elastic force" and "electromagnetic force." Most of these methods and devices are passive.
In instruments utilizing elastic force, the amount of force applied is measured in terms of displacement and deformation of a known elastic member, and is the property exploited in most "force transducing" methods. These instruments, based on the equilibrium of the force on or by the object and the elastic force, are generally referred to as dynamometers, more commonly known as load cells. The working principle of the dynamometer is based on Hooke's law, which is the linear proportional relationship of stress and strain. The strain of the elastic element is measured when subjected to the load of an object, with the force exerted on or by the object being determined. The elastic element ranges from the following: helical elements subjected to compression or twisting (springs); solid or hollow columns working in compression or tension (load cells); circular, square, and flattened rings, working in compression-and-bending or in tension-and-bending; tension bars; shear bars; etc.
There are other systems of force transducers that also use linear functions, like Hooke's law, but in which the force applied modifies some other measurable property of the transducer. Piezoelectric material falls into this category in that, when deformed, it yields an electric charge proportional to the deformation. If the deformation is maintained, however, the charge will gradually decrease. Therefore, piezoelectric transducers are not suitable for static loads. Instead, these materials are utilized in situations where dynamic force characteristics are required.
In instruments utilizing electromagnetic force, the amount of force applied is measured in terms of electric current. Electromagnetic forces are generated by a magnetic field of an electric charge in motion. The generation of electromagnetic forces has been applied in construction of electromagnetic compensation transducers, in which the force of an object is balanced by the electromagnetic force generated by a coil assembly in a magnetic field. The force is determined by the magnitude of the electric current necessary to bring the system to equilibrium.
In the initial stages of the present development project, a voice coil actuator was arranged to receive a direct force by applying an item to be weighed directly to it and attempting to measure the current required to create an equal but opposite force. This was found to have inherent complications that prevent it from being a useful, accurate system. These complications include excessive sensitivity to environmental vibrations even if a vibration isolator is used, inability to keep in calibration, and measurement interference by lead wires to the coil, to name a few. This type of system was therefore discarded as flawed, with further development leading to the invention herein.
A subsequent search revealed a system with an additional complication of a mechanical diaphragm in the form of an annular blade spring. Specifically, U.S. Pat. No. 4,802,541 to Bator et al. describes a weighing scale employing a tray supported on annular blade springs and having a conductive coil movable therewith relative to a fixed magnet. A control system senses a change in position of the tray with applied weight, and causes sufficient current to the coil to return the coil and tray to the original position. This proposed apparatus is considered to have significant shortcomings, however, among which are the non-uniformity and variable characteristics of springs which are used, the undesirable effects of moving coil lead wires on the weighing action, and the limited capacity of vibration isolators to actually isolate the apparatus from low frequency vibration found, for example, in production facilities. Present units employ passive isolation, as contrasted with detecting motion and actively responding with a control system.
Achievable accuracies are usually specified by manufacturers of force transducers. Typical sensors based on elastic elements can achieve approximately 0.25% of full scale, and the best ones are accurate up to 0.05%. Force transducers utilizing electromagnetics are available with accuracies up to 0.00004% of scale. Usually to achieve such accuracy requires a weighing response time of &gt;1.0 second. Certain factors can adversely influence accuracy, however. It was determined by the inventors herein that low frequency vibrations in the range of below about 2 Hertz are highly disruptive of a voice coil actuator system such as in U.S. Pat. No. 4,802,541, since the isolator does not isolate the coil from such vibrations. Such low frequency vibration is common in production facilities, e.g., from stamping presses, so that it presents a significant problem. Also, it has been determined that the drag or weight effects of wire leads on a movable coil disrupt accurate readings. If these effects were not present, the achievable accuracies of "off the shelf" force transducers could easily meet the accuracy requirement, but their response time does not meet the speed requirement. Manufacturers do not generally give specifications for a required time to achieve the given accuracy. Speed claims that are made by manufacturers of force transducers are generally of the nature "responds in about one second with stable readings." The reason for this vagueness is that achieving a high level of speed is dependent on having a controller tuned very accurately for the particular application. Available force transducers are not easily tuned by the user to meet a strict speed requirement for a particular application.
A suitable electromagnetic actuator for an active force transducer application is what is commonly known as a voice coil actuator (VCA). VCAs have been utilized in many advanced motion control systems requiring high acceleration, high frequency actuation and linear force output. In its simplest form, a linear VCA, i.e., linear motor, is a tubular coil of wire located within a radially oriented magnetic field. The field is produced by permanent magnets. The inner core of the magnet along the axial centerline of the coil is used to complete the magnetic circuit. The force generated axially upon the coil when current flows through the coil will produce relative motion between the magnet assembly and the coil. The electromagnetic conversion mechanism of the VCA is governed by the Lorentz Force principle. The direction of current flow through the coil dictates the direction of the axial force, while the magnitude of the force is proportional to current. For precise servo control applications utilizing VCAs, it is required to have a control and power amplifier circuitry and also a feedback device for closed-loop control. Many types of position, velocity, and force transducers can function as feedback devices, as is known. In order to eliminate the potential of static friction, the coil must be centered within the magnet/core gap throughout the entire stroke. It is not necessary to center the coil precisely; however, contact between the coil assembly and magnet assembly must be avoided.
Because the force transducer would be expected to be located in a manufacturing environment, ambient vibrations including low-frequency and high-frequency vibrations from manufacturing operations will be present and need to be precluded from significantly affecting accuracy of the output readings.
Conventional beam style force transducers generally employ relatively large beams which tend to have a large mass and can experience undesirable beam bending moments. In addition, known beam style force transducers can experience excessive beam pivot friction which degrades the accuracy of the force transducers. Further, such conventional beam style transducers often require precise position placement of an object to be weighed on the beam to maintain accuracy.